= Solution
The three regions represent competing pathways whose leaving-group sensitivities differ.
* \b[Region 1, $\beta_{\rm lg}=-1.33$:] good leaving groups permit <E1cB mechanism>. OH$^-$ first deprotonates the para phenol. The phenoxide lone pair then pushes through the ring, with loss of ArO$^-$ from the acyl group, giving a <quinoid ketene>, schematically $\mathrm{O=C_6H_4=C=O}$. Water/<hydroxide> traps the ketene and proton transfers restore the aromatic hydroxybenzoate. Rate-limiting fragmentation has extensive C–OAr cleavage, hence strong dependence on leaving-group basicity.
* \b[Region 2, $\beta_{\rm lg}=-0.35$:] as ArO$^-$ becomes a worse leaving group, elimination slows steeply and ordinary <hydroxide> addition to neutral C becomes competitive/dominant. The tetrahedral acyl-substitution pathway is much less sensitive to leaving-group p$K_a$.
* \b[Region 3, $\beta_{\rm lg}=-0.20$:] <hydroxide> attacks the deprotonated phenoxide <ester> C$^-$. Its low electrophilicity and anion-anion interaction make addition slow. The leaving group is little involved in that addition transition state. Combined with prior <deprotonation>, this is the third-order term.
The bend in the upper plot is a crossover of mechanisms, not a discontinuous change in the identity of the leaving group. The lower plot is the separate hydroxide-on-phenoxide pathway.
<Model experiments implicate a benzoquinoneketene intermediate in LCP synthesis by Jeffrey M. Robertson and colleagues>{ref}
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-4-hydrolysis-routes.png]
{title=Parallel acyl-substitution and conjugate-base elimination pathways}
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